Up to now, a vehicular electronic key system has been known in which an in-vehicle device mounted on a vehicle and a mobile device carried by a user perform an authentication process by a wireless communication, and the in-vehicle device executes a vehicle control such as vehicle door locking and unlocking and engine starting based on a success of the authentication process. In the vehicular electronic key system of that type, an arrival range of a radio signal transmitted by the in-vehicle device is limited to a short distance around the vehicle. This is because a situation in which the in-vehicle device carries out the wireless communication with the mobile device is limited to a case in which the mobile device is present in the vicinity of the vehicle.
However, the vehicular electronic key system described above causes concern about a relay attack that a third party with malicious intent indirectly realizes a communication between the mobile device and the in-vehicle device with the use of a repeater, thereby realizing unauthorized authentication of the mobile device by the in-vehicle device. If the relay attack succeeds, the vehicle control such as unlocking the vehicle door or starting the engine is executed even though an authorized user does not intend to execute the vehicle control.
On the other hand, various configurations for preventing such a relay attack have also been proposed. An in-vehicle device of a vehicular electronic key system disclosed in, for example, Patent Literature 1 transmits a radio signal including two types of power levels having a high level and a low level, and a mobile device sequentially detects a strength of a received signal (so-called RSSI: received signal strength indication). The mobile device returns the signal to the in-vehicle device only when the mobile device detects a change in strength of a predetermined level or higher in the received signal.
The relay attack countermeasure disclosed in Patent Literature 1 is a technology focused on a fact that a repeater used for the relay attack amplifies a radio wave of a frequency band to be relayed to a certain output level and transfers the amplified radio wave. In other words, when the radio wave transmitted from the in-vehicle device is relayed by the repeater, a level difference of the received signal which is to be originally detected is not detected. In other words, according to the configuration of Patent Literature 1, the mobile device can restrict a response from returning to the signal from the in-vehicle device relayed by the repeater. Naturally, unless the response from the mobile device is returned, the authentication is not established (that is, fails).
In general, because the strength of the signal attenuates according to a propagation distance, the RSSI detected by a receiver side is larger as a distance from a signal transmission source is smaller. Therefore, the mobile device receives the signal transmitted from the in-vehicle device with a larger strength as a distance between the mobile device and an antenna (hereinafter referred to as a vehicle-side transmission antenna) provided for the mobile device to transmit the signal is shorter.